Refrigeration systems e.g. in supermarkets and cooling stores are generally losing their refrigerant charge at a slow rate. Typical numbers rate 10-30% of the total charge per year. This eventually leads to failure due to a lack of refrigerant.
To compensate for the lost refrigerant and to avoid failures, fixed service intervals are typically defined.
The refrigerant is partly present in the form of liquid and partly in form of vapor. The amount of liquid in the evaporators and the heat rejecting heat exchanger varies dramatically depending on operating conditions and particularly on temperatures, pressures, and load.
The refrigerant migrates during normal operation. Most RVCS systems will therefore comprise a receiver that stores an amount of refrigerant to accommodate the variations in charge in the other parts of the plant. This makes it virtually impossible to estimate the charge under normal running conditions.
Similar considerations apply in other technically similar systems, e.g. in heat pump systems for domestic heating and in air condition systems etc. Herein, use of the term RVCS system encompass any kind of compressor and refrigerant based system where thermal energy is exchanged between a warm and a cold side by use of a compressor and a refrigerant. Such systems are sometimes referred to as refrigerant vapor compression systems.
There is great interest in the market for early detection of low charge conditions which would make it possible to schedule re-charging before the failure actually occurs. This may reduce the amount of failures and prevent the need for emergency service calls.
During refilling, the service personnel refill the system with a roughly estimated amount of refrigerant. Current practice is very imprecise, and typically focused on avoiding over-filling the system since this may cause costly breakdown of the compressor if liquid refrigerant escapes from the evaporator.